ANIMAL MECHANISM. 135 



to prevent the immediate action of the water on the eye, 

 but also to discover their prey. But as the light loses 

 more of its power on passing through water, than in 

 passing through air, and is still more weakened in its 

 progress through the nictitating membrane, it follows 

 that owing to this membrane, vision must be less distinct 

 under the water than in the air. 



THE REASON WHY MAN CANNOT SEE UNDER WATER. 



A man under water, sees objects as a very aged person 

 sees through a concave glass, placed close to the eye. 

 The fish is long-sighted under water and man is short- 

 sighted. If he uses spectacles, whose convexity is just 

 double the convexity or equal in convexity on both 

 sides to the cornea of his own eye, he will see under 

 water. The necessity of this is obvious ; the aqueous 

 humor is of the same density with the water, and there 

 cannot, therefore, be any refraction of the rays in passing 

 from the water into the land-seeing eye. 



Euclid and other distinguished ancients, contended, 

 and, indeed, supposed that vision was occasioned by the 

 emission of rays from the eye to the object. He thought 

 it more natural to suppose that an animate substance 

 gave an emination, than that the inanimate body did. In 

 1560, the opinion that the rays entered the eye, was 

 established. Kepler, in 1600, snowed, geometrically, how 

 the rays were refracted through all the humors, so as to 

 form a distinct picture on the retina ; and he also demon- 

 strated the effect of glasses on the eyes. 



IN WHAT MANNER DOES THE EYE ADAPT ITSELF TO THE 

 DISTANCE OP OBJECTS 1 



No one has satisfactorily answered this question. One 

 philosopher supposes the eye is at rest, when we examine 

 a distant object, as a mountain, the spire of a church, or 

 a landscape, but, that in the act of seeing near objects, 

 there is an effort. It has been supposed that this effort 

 is the action of the straight or recti muscles, exhibited in 

 the first plan of the cordage of the eye, compressing the 

 globe, so equally, as to elongate the eye, and lengthen 

 the axis, so much, as to favor the union of the pencils of 



